It has shown that the carboxyl terminal tyrosine of the alpha-chain of tubulin turns over very rapidly in cell culture compared to the turnover time of tubulin itself. The half-life of the carboxyl terminal tyrosine is 37 minutes in myogenic cell cultures while that of tubulin is greater than two days. Functional microtubules are required for the rapid turnover of the carboxyl terminal tyrosine, and the turnover time may be correlated with the activity of tubulin:tyrosine ligase. The significance of this rapid turnover time for differentiating cells, in which the fibrillar elements undergo continuous reorganization, will be investigated. Nerve growth factor, hormones and drugs which influence these cytoskeletal structures will be employed for this purpose. These studies will be carried out both with cells in culture and, in vivo, in the developing frog.